The research proposed for the next year is aimed at understanding the mode(s) of action of chicken thymus derived suppressor lymphocytes (Ts). Several questions are currently being asked, a) What are the target antigens that are recognized by Ts? b) When during ontogeny do the Ts target antigens become expressed? c) What are the histocompatibility requirements for Ts activity? d) Are viable Ts required for the suppression of immunoglobulin synthesis or can soluble Ts-derived factors produce suppression? e) What is the functional capability of the B-lymphocytes present in recipients made agammaglobulinemic by transplantation of Ts cells?